Broadband loop antenna



Patented June 16 195 3 UNITED STATES PATENT OFFICE B OA AND LOOP ANTENNASidney Frankel, Bergensfield, N. J., assignor to International StandardElectric Corporation, New :York, 'N. X, a nQnati n f .Dslamere:

fi lm eai nt 9$e a1.N9- W9 9 "7 Claims. 1

This invention relates to antennae and is particularly directed toimprovements in loop antennae having high radiation conductance andbroadband characteristics in the ultrahigh frequency range. are adaptedfor 'nondirection'al -br'o'adcasting.

It is desirable in ultrahigh frequency b'roadcasting to effic'ientlyload the generator of the energy to be radiated as well as to preventfrequency selectivity of the radiator -system over ayw-ide range offrequencies. -Unfortunately, both these conditions have not heretoforebeen realized in one antenna structure. While the reactive components ofthe load'ma-y-b'e tuned out by the addition of reactances ofcompensating signs, the resulting Q is high and broadbandcharacteristics are sacrificed.

An object of this invention is an antenna that has both broadbandcharacteristics and high ra=- diation conductance.

A more specific object of this invention is a loop-type antenna that isnondirectional.

A still more specific object of this invention is a nondirectio'nal loopantenna of simple, rugged, and easy-to-"nake construction.

The objects of this invention are realized by a longitudinally slottedsheet metal cylinder with the conductors of a coaxial transmission lineextending into one end and through the cylinder.

The edges of the cylinder slot are connected; re-

spectively, with the inner .and outer conductors and the annular spacebetweenthe cylinder and outer conductor constitutes a tunable cavityresonator electrically in parallel-with the impedance at the slot. n

The above-mentioned and other features and objects of this invention and"the manner ofattaining them will become apparent and the inventionitself will bebest understood by reference to the following descriptionof --preferred embodiments of the invention taken in conjunction withthe accompanying drawing,"wherein:

Fig. 1 is a perspective view, partly in section, of a single slotantenna structure of this invention;

Fig. 2 is an end view of the structure of Fig. 1;

Fig. 2a is a circuit diagram of the principal electrical parts of thestructure of Fig. 2; and

Figs. 3 and 4 are end views of multi-cavity modifications of the antennaof Fig. 1.

The antenna of this invention comprises a symmetrical smooth sheet metalcylinder 1 closed at one end'with plate 2 having a longitudinal slot 4extending from end-to-end. A coaxial transmission line is telescopedinto said cylinder through one end, inner conductor 5 and outerconductor The antennae of this invention 6 extending through to the endplate. The high frequency voltage between the inner and outer conductorsare applied, respectively, to opposite edges of the slot i by radiallydisposed 'vanes 1 and 8. The edges of vane I are joined'in'c'o'ntinuousline contact to one edge of the cylinder slot and the outer conductor,While the vane i8 extends centrally through a window in the outerconductor Band joins the inner conductor t'o' the other edge of theslot. Openings 9 and I0 between the vane 8 and either side of the windowmay-be predetermined as hereinafter described.

in Fig. 2a. It has been found that the imped ance Z of the cavityresonator presented at the input opening II] to the transmission linemay be adjusted to capacitive or inductive value for any appliedfrequency merely by tuning the cavity resonator to resonance at afrequency above or below the applied frequency. such tuning is mostconveniently'effected by moving the partition l2 toward oraway from theopening 10. It is found -ifurther ,,that the 'impe'dance'presented bythe outside slot 4 to the transmission line is capacitive -C andresistive R only, the absence of inductive' ilnpedance being expected Iwhen it is recalled the antenna cylindcr is closed at one end and canlink 'withlno electromagnetic lines of force. That -'is, radi'o'frequency current flows radially over the opposed faces of the vanes land 8 and from one edgeiof th'e' sl'ot to'the other along the'ou'tersurf-ace of the cylinder, and since the cylinder sides and end areimpervious to such currents there can be no self induced voltages. Now,according to an important feature of this invention, the capacitivereactance of the loop circuit may be precisely balanced out by an equaland parallel inductive reactance that can be obtained at the opening I!)to the cavity by adjustment of the tuning partition l2. The load thuspresented to the transmission line can be made entirely resistive andbecause the conductor and dielectric losses are small the largestportion of the resistance is attributable to radi' ation. For thesmaller range of spacings between 7 and 8, the shunt capacitance variesin- 3 versely with the spacing. The efiiciency of radiation of thisantenna is high.

More uniform radiation, increased radiation conductance, and stillbroader band characteristics may be obtained by employing a largernumber of slots. In Fig. 3, for example, four equally spaced slots I3, Il, I5 and it are shown, although a larger or smaller number may be usedif desired. As in Fig. 1 the two edges of each slot is joined by vanes 1and 8 to the outer and inner conductors, respectively, of thetransmission line. Each of the wider vanes 8 extends through, withoutcontacting, the windows the outer conductor so that the fields withinthe line are open to both the antenna slots l3-!6 and'to cavityresonators ll. Each resonator is tuned by the short-circuit partition[2. In case the number of segments of the antenna cylinder, for theoperating frequency, is large, and the circumferential distance betweenslots is too small to accommodate the full length. of the cavityresonators, the resonators may be folded as shown in Fig. 4. By theplacement of sheet metal walls a cavity of desired length may be coiledwithin the limits of each segment. It is suggested in Fig.4 thatvthevanes and curved wall portions of each segment be fashioned from singlepieces of sheet metal, copper being one metal that has good conductivityand is easily bent.

While there has been described above the principles of this invention inconnection with sp e cific apparatus, it is to be clearly understoodthat this description is made only by way of example and not as alimitation to the scope of this in vention.

What is claimed is: 1. An antenna comprising a coaxial transmissionline, a longitudinally slotted cylinder encircling and coaxial with theconductors of said transmission line, the outer conductor of said linehaving a slot in registry and coextensive with the slot of saidcylinder, the edges of the cylinder defining the slot being joinedthroughout the length of the cylinder slot to the inner conductor andouter conductor, respectively, of said transmission line.

the space inside said outer conductor, and a movable vane in saidannular space to adjust the dimensions thereof.

4. The combination comprising a transmission line with coaxialconductors adapted to conduct high frequency energy, a loop antennacomprising a sheet metal cylinder telescoped over and concentric withthe outer conductor of said line and provided with at least onelongitudinal slot, the outer conductor of said line being slotted .-overits length within said cylinder, means con- 2. An antenna systemcomprising a sheet metal 5 cylinder and, concentric conductors of atransmission line, said cylinder and conductors being coaxial and beingprovided with. registering longitudinal slots in the cylnder and outerconductor,

a first radialvane joining one edge of the cylinder defining the slottherein to one edge of the outer conductor defining the slot therein, asecond radial vane extending through the outer conductor slot andjoining the other edge of the cylinder defining'the .slot therein to theinner conductor.

3. An'antenna system as defined in'claim 2 in which the annular spacebetween said cylinder 7 and outer conductor is in communication withpling said transmission line and said antenna through said slots forimpressing high frequency voltage across the edges of said slotfrom theelectric field within said transmission line and for exciting theannular space between the cylinderand said outer conductor of the line5. The combination of a loop antenna and transmission line, said linecomprising coaxial conductors provided with windows in the outerconductor and said antenna comprising a metal cylinder telescoped oversaid line and provided with aplurality of longitudinal slots in saidcylin der registering and coextensive with said windows in the outerconductor of said line, a vane extende ing radially from the innerconductor centrally through each of said windows to one edge of a saidcylinder defining each of said slots, and vanes joining the oppositeedge of said cylinder defining each slot to one edge in said outerconductor defining each window.

6. The combination as defined'in claim 5 in.

outer cylinder being provided with a slot, the

edges of said cylinder defining the slot being electrically connected tothe inner cylinders, the first annular space between the outer twocylinders being provided with a communicating opening into the secondspace between the inner two cylin ders, and a radial short circuitingvane in said first space and adjustable in a circumferential directionalong said first space.

. SIDNEY FRANKEL.

References Cited in the file of this patent UNITED STATES PATENTS NameDate Blumlein Apr. 15, 194i Barrow Aug. 26, 1941 Lindenblad Jan. 14,1947 Riblet Jan. 6, 1948 Buchwalter Nov. 30, 1948 Chu July 26, 1949Hansen Oct. 11, 1949 Masters 1 June 6, 1950 Number

